共查询到20条相似文献，搜索用时 15 毫秒
1.
Kronrod E. V. Kronrod V. A. Kuskov O. L. Nefedyev Yu. A. 《Doklady Earth Sciences》2018,483(1):1475-1479
Doklady Earth Sciences - The bulk composition of the silicate Moon (crust + mantle, BSM) is determined on the basis of inversion of gravitational and seismic data. It is shown that the mantle... 相似文献
2.
Re-Os同位素体系是理解月球强亲铁元素的分布规律和示踪月球的后期增生历史的重要手段。目前人们对月球物质Re-Os同位素成分的了解还是十分有限的,已有的Re-Os同位素数据显示一些能代表月幔成分特征的月海玄武岩具有很低的Re和Os的浓度,以及类似于球粒陨石的187Os／188Os成分特征,而月球火山玻璃和月壤等表现出相对高的Re-Os丰度和相对富放射成因Re-Os同位素成分。一般认为月球月幔的Re、0s和其他强亲铁元素相对球粒陨石是非常亏损的,而地球地幔则具有相对较高的强亲铁元素丰度（0．008倍CI球粒陨石的丰度）。新的Re-Os同位素结果证明月幔确实是亏损的,但是月球和地球在太阳系演化的较晚时期都有外来的球粒陨石物质的大量加入,即后期增生（late accretion）过程,导致了月球和地球上部物质（如月球火山玻璃、月壤等）相对地富集Os同位素和强亲铁元素,这些外来物质的后期增生可能是长期和持续的,增生过程主要发生在3．9～4．4Ga。但目前仍不清楚后期增生的陨石物质是被逐渐加入的,还是在一个相对较短的时期大量加入的,因此尚需对更多的月球物质做进一步的Re-Os同位素和强亲铁元素成分的研究。 相似文献
3.
月球的化学演化 总被引：2，自引：0，他引：2
月球是一个发生了化学分异的星球,它由月壳、月幔±一个小的金属月核组成。大量观察事实显示月球曾经有过岩浆洋,岩浆洋的结晶分异主导了月球的化学演化。目前主流观点认为,月球是在太阳系演化的早期,至少45亿年前,一个火星大小的星球,与即将完成原始吸积的地球胚胎发生偏心撞击,造成地球的熔融,形成岩浆洋,飞溅出来的物质迅速吸积形成绕地球运动的月球,并且在月球上形成了全球规模的岩浆洋,进而发生了结晶分异。,由于月球上没有海洋和板块俯冲,岩浆洋分异是其化学演化的主要途径。月球岩浆洋的80％～85％在大撞击后的100Ma内已经固化,这可能是由于月球体积小、表面没有大气包裹所致。月球极贫水,因此在岩浆结晶过程中斜长石首先结晶。斜长石由于密度小于玄武质岩浆而漂浮在岩浆洋的表层,橄榄石等密度大的矿物则堆积在岩浆洋的底部。随着结晶分异的进行,残余岩浆不断富集不相容元素,包括K、U等放射性元素;与此同时,密度较大的钛铁矿开始结晶,造成高钛堆晶岩密度大于其下的橄榄石堆晶岩的不稳定结构,进而发生月幔翻转,引发一系列岩浆活动,进而形成月球上特有的镁质系列、碱质系列等岩石。由于月球氧逸度较低,Eu主要以＋2价形式存在,因此斜长石高度富集Eu,相应地除高地斜长岩外,其他岩石均表现为Eu高度亏损的特点。与此同时,Re在低氧逸度下表现为强亲铁元素的特点,Re／Os在月球岩浆过程中不发生分异。月球的体积远小于地球,因而其演化时间远远短于地球,很多原始的分异被完整地保留下来。因此月球的化学演化是类地行星早期演化过程的“化石”,尽管与现代的地球存在较大差异,但是对于认识地球早期演化具有借鉴意义。 相似文献
4.
Tectonomagmatic evolution of the Earth and Moon 总被引：1，自引：0，他引：1
The Earth and Moon evolved following a similar scenario. The formation of their protocrusts started with upward crystallization
of global magmatic oceans. As a result of this process, easily fusible components accumulated in the course of fractional
crystallization of melt migrating toward the surface. The protocrusts (granitic in the Earth and anorthositic in the Moon)
are retained in ancient continents. The tectonomagmatic activity at the early stage of planet evolution was related to the
ascent of mantle plume of the first generation composed of mantle material depleted due to the formation of protocrusts. The
regions of extension, rise, and denudation were formed in the Earth above the diffluent heads of such superplumes (Archean
granite-greenstone domains and Paleoproterozoic cratons), whereas granulite belts as regions of compression, subsidence, and
sedimentation arose above descending mantle flows. The situation may be described in terms of plume tectonics. Gentle uplifts
and basins (thalassoids) in lunar continents are probable analogues of these structural elements in the Moon. The period of 2.3–2.0 Ga ago was a turning point in the
tectonomagmatic evolution of the Earth, when geochemically enriched Fe-Ti picrites and basalts typical of Phanerozoic within-plate
magmatism became widespread. The environmental setting on the Earth’s surface changed at that time, as well. Plate tectonics,
currently operating on a global scale, started to develop about ∼2 Ga ago. This turn was related to the origination of thermochemical
mantle plumes of the second generation at the interface of the liquid Fe-Ni core and silicate mantle. A similar turning point
in the lunar evolution probably occurred 4.2–3.9 Ga ago and completed with the formation of large depressions (seas) with thinned crust and vigorous basaltic magmatism. Such a sequence of events suggests that qualitatively new material previously
retained in the planets’ cores was involved in tectonomagmatic processes at the middle stage of planetary evolution. This
implies that the considered bodies initially were heterogeneous and were then heated from above to the bottom by propagation
of a thermal wave accompanied by cooling of outer shells. Going through the depleted mantle, this wave generated thermal superplumes
of the first generation. Cores close to the Fe + FeS eutectics in composition were affected by this wave in the last turn.
The melting of the cores resulted in the appearance of thermochemical superplumes and corresponding irreversible rearrangement
of geotectonic processes. 相似文献
5.
The evolution of terrestrial planets (the Earth, Venus, Mars, Mercury, and Moon) was proved to have proceeded according to
similar scenarios. The primordial crusts of the Earth, Moon, and, perhaps, other terrestrial planets started to develop during
the solidification of their global magmatic “oceans”, a process that propagated from below upward due to the difference in
the adiabatic gradient and the melting point gradient. Consequently, the lowest melting components were “forced” toward the
surfaces of the planets in the process of crystallization differentiation. These primordial crusts are preserved within ancient
continents and have largely predetermined their inner structure and composition. Early tectono-magmatic activity at terrestrial
planets was related to the ascent of mantle plumes of the first generation, which consisted of mantle material depleted during
the development of the primordial crusts. Intermediate evolutionary stages of the Earth, Moon, and other terrestrial planets
were marked by an irreversible change related to the origin of the liquid essentially iron cores of these planets. This process
induced the ascent of mantle superplumes of the second generation (thermochemical), whose material was enriched in Fe, Ti,
incompatible elements, and fluid components. The heads of these superplumes spread laterally at shallower depths and triggered
significant transformations of the upper shells of the planets and the gradual replacement of their primordial crusts of continental
type by secondary basaltic crusts. The change in the character of the tectono-magmatic activity was associated with modifications
in the environment at the surface of the Earth, Mars, and Venus. The origin of thermochemical mantle plumes testifies that
the tectono-magmatic process involved then material of principally different type, which had been previously “conserved” at
deep portions of the planets. This was possible only if (1) the planetary bodies initially had a heterogeneous inner structure
(with an iron core and silicate mantle made up of chondritic material); and (2) the planetary bodies were heated from their
peripheral toward central portions due to the passage of a “thermal wave”, with the simultaneous cooling of the outer shells.
The examples of the Earth and Moon demonstrate that the passage of such a “wave” through the silicate mantles of the planets
was associated with the generation of mantle plumes of the first generation. When the “wave” reached the cores, whose composition
was close to the low-temperature Fe + FeS eutectic, these cores started to melt and gave rise to superplumes of the second
generation. The “waves” are thought to have been induced by the acceleration of the rotation of these newly formed planets
due to the decrease of their radii because of the compaction of their material. When this process was completed, the rotation
of the planets stabilized, and the planets entered their second evolutionary stage. It is demonstrated that terrestrial planets
are spontaneously evolving systems, whose evolution was accompanied by the irreversible changes in their tectono-magmatic
processes. The evolution of most of these planets (except the Earth) is now completed, so that they “dead” planetary bodies. 相似文献
6.
地球排气作用——建立整体地球科学的一条统纲 总被引：22，自引：2，他引：20
通过研究发现 ,现在可以用地球排气作用作为一条统纲 ,把原来分散的地球科学各个分支学科和谐地串联成为一个整体地球科学巨系统。地球排气的实质是幔汁 (HACONS超临界态流体 )从深部自发向上、向外的辐射排放 ,结果形成了地球内外的 5个气圈和气体地球动力学 ,幔汁上涌后通过碱交代作用产生蚀变和岩浆并促使原来为固体的上地幔、地壳发生溃变 ,在地球的 3个基本动力 ,即重力、地球自转变速、圈层旋转差速和天体 (主要是月球 )潮汐力联合作用的大前提下诱发出大地构造运动、地球演化和自然灾害。没有幔汁活动就没有壳幔运动。笔者初步认为 ,对现有流行的众多理论和结论都需要重新加以怀疑和再审鉴。否则 ,地球科学难以向前发展。我们今后不仅需要单科的或局域的岩石学家、地球化学家、大地构造学家、气象学家、海洋学家等 ,另外还需要有一大批整体地球科学家。 相似文献
7.
Melting relations of primitive peridotite were studied up to 25 GPa. The change of the liquidus phase from olivine to majorite occurs at 16 GPa. We confirmed the density crossover of the FeO-rich peridotite melt and the equilibrium olivine (Fo90) at 7 GPa. Sinking of equilibrium olivine (Fo95) in the primitive peridotite melt was observed up to 10 GPa. The compression curves of FeO-rich peridotitic and komatiite melts reported in this and earlier work suggest that the density crossover in the Earth's mantle will be located at 11–12 GPa at 2000°C, consistent with an previous estimation by C.B. Agee and D. Walker.
The density crossover can play a key role in the Moon and the terrestrial planets, such as the Earth, Venus and Mars. Majorite and some fraction of melt could have separated from the ascending diapir and sunk downwards at the depths below the density crossover. This process could have produced a garnet-rich transition zone in the Earth's mantle. The density crossover may exist in the FeO-rich lunar mantle at around the center of the Moon. The density crossover which exists at the depth of 600 km in the Martian mantle plays a key role in producing a fractionated mantle, which is the source the parent magmas of the SNC meteorites. 相似文献
8.
The discrepancy between the impact records on the Earth and Moon in the time period, 4.0-3.5 Ga calls for a re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian records. Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original satellites of the Earth and their debris in the time period about 4.0-3.5 Ga. The effects on Earth of such encounters could have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0-3.5 Ga, when the heavy and declining lunar bombardment took place. The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion--existing models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere. Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life. 相似文献
9.
S. König C. Münker H. Paulick M. Lagos A. Büchl 《Geochimica et cosmochimica acta》2011,75(8):2119-17609
During silicate melting on Earth, W is one of the most incompatible trace elements, similar to Th, Ba or U. As W is also moderately siderophile during metal segregation, ratios of W and the lithophile Th and U in silicate rocks have therefore been used to constrain the W abundance of the Earth’s mantle and the Hf-W age of core formation. This study presents high-precision W concentration data obtained by isotope dilution for samples covering important silicate reservoirs on Earth. The data reveal significant fractionations of W from other highly incompatible lithophile elements such as Th, U, and Ta. Many arc lavas exhibit a selective enrichment of W relative to Th, U, and Nb-Ta, reflecting W enrichment in the sub-arc mantle via fluid-like components derived from subducting plates. In contrast, during enrichment by melt-like subduction components, W is generally slightly depleted relative to Th and U, but is still enriched relative to Ta. Hence, all arc rocks and the continental crust exhibit uniformly low Ta/W (ca. 1), whereas W/Th and W/U may show opposite fractionation trends, depending on the role of fluid- and melt-like subduction components. Further high-precision W data for OIBs and MORBs reveal a systematic depletion of W in both rock types relative to other HFSE, resulting in high Ta/W that are complementary to the low Ta/W observed in arc rocks and the continental crust. Similar to previous interpretations based on Nb/U and Ce/Pb systematics, our Ta/W data confirm a depletion of the depleted upper mantle (DM) in fluid mobile elements relative to the primitive mantle (PRIMA). The abundance of W in the depleted upper mantle relative to other immobile and highly incompatible elements such as Nb and Ta is therefore not representative of the bulk silicate Earth. Based on mass balance calculations using Ta-W systematics in the major silicate reservoirs, the W abundance of the Earth’s primitive mantle can be constrained to 12 ppb, resulting in revised ratios of W-U and W-Th of 0.53 and 0.14, respectively. The newly constrained Hf-W ratio of the silicate Earth is 25.8, significantly higher than previously estimated (18.7) and overlaps within error the Hf-W ratio proposed for the Moon (ca. 24.9). The 182Hf-182W model age for the formation of the Earth’s core that is inferred from the 182W abundance and the Hf/W of the silicate Earth is therefore younger than previously calculated, by up to 5 Myrs after solar system formation depending on the accretion models used. The similar Hf/W ratios and 182W compositions of the Earth and the silicate Moon suggest a strong link between the Moon forming giant impact and final metal-silicate equilibration on the Earth. 相似文献
10.
《Comptes Rendus Geoscience》2007,339(14-15):917-927
Plate tectonics shaped the Earth, whereas the Moon is a dry and inactive desert, Mars probably came to rest within the first billion years of its history, and Venus, although internally very active, has a dry inferno for its surface. Here we review the parameters that determined the fates of each of these planets and their geochemical expressions. The strong gravity field of a large planet allows for an enormous amount of gravitational energy to be released, causing the outer part of the planetary body to melt (magma ocean), helps retain water on the planet, and increases the pressure gradient. The weak gravity field and anhydrous conditions prevailing on the Moon stabilized, on top of its magma ocean, a thick buoyant plagioclase lithosphere, which insulated the molten interior. On Earth, the buoyant hydrous phases (serpentines) produced by reactions between the terrestrial magma ocean and the wet impactors received from the outer solar system isolated the magma and kept it molten for some few tens of million years. The planets from the inner solar system accreted dry: foundering of wet surface material softened the terrestrial mantle and set the scene for the onset of plate tectonics. This very same process also may have removed all the water from the surface of Venus and added enough water to its mantle to make its internal dynamics very strong and keep the surface very young. Because of a radius smaller than that of the Earth, not enough water could be drawn into the Martian mantle before it was lost to space and Martian plate tectonics never began. The radius of a planet is therefore the key parameter controlling most of its evolutional features. 相似文献
11.
M.I. Kuzmin V.V. Yarmolyuk A.B. Kotov N.A. Goryachev 《Russian Geology and Geophysics》2018,59(12):1535-1547
The paper is focused on the evolution of the Earth starting with the planetary accretion and differentiation of the primordial material (similar in composition to CI chondrites) into the core and mantle and the formation of the Moon as a result of the impact of the Earth with a smaller cosmic body. The features of the Hadean eon (ca. 4500–4000 Ma) are described in detail. Frequent meteorite-asteroid bombardments which the Earth experienced in the Hadean could have caused the generation of mafic/ultramafic primary magmas. These magmas also differentiated to produce some granitic magmas, from which zircons crystallized. The repeated meteorite bombardments destroyed the protocrust, which submerged into the mantle to remelt, leaving refractory zircons, indicators of the Early Earth’s geologic conditions, behind.The mantle convection that started in the Archean could possibly be responsible for the Earth’s subsequent endogenous evolution. Long-living deep-seated mantle plumes could have promoted the generation of basalt-komatiitic crust, which, thickening, could have submerged into the mantle as a result of sagduction, where it remelted. Partial melting of the thick crust, leaving eclogite as a residue, could have yielded tonalite-trondhjemite-granodiorite (TTG) melts. TTG rocks are believed to compose the Earth’s protocrust. Banded iron bodies, the only mineral deposits of that time, were produced in the oceans that covered the Earth.This environment, recognized as LID tectonics combined with plume tectonics, probably existed on the Earth prior to the transitional period, which was marked by a series of new geologic processes and led to a modern-style tectonics, involving plate tectonics and plume tectonics mechanisms, by 2 Ga. The transitional period was likely to be initiated at about 3.4 Ga, with the segregation of outer and inner cores, which terminated by 3.1 Ga. Other rocks series (calc-alkaline volcanic and intrusive) rather than TTGs were produced at that time. Beginning from 3.4-3.3 Ga, mineral deposits became more diverse; noble and siderophile metal occurrences were predominant among ore deposits. Carbonatites, hosting rare-metal mineralization, could have formed only by 2.0 Ga. From 3.1 to 2.7 Ga, there was a period of “small-plate” tectonics and first subduction and spreading processes, which resulted in the first supercontinent by 2.7 Ga. Its amalgamation indicates the start of superplume-supercontinent cycles.Between 2.7 and 2.0 Ga, the D″ layer formed at the core-mantle interface. It became a kind of thermal regulator for the ascending already tholeiitic mantle plume magmas. All deep-seated layers of the Earth and large low-velocity shear provinces, called mantle hot fields, partially melted enriched EM-I and EM-II mantles, and the depleted recent asthenosphere mantle, which is parental for midocean-ridge basalts, were finally generated by 2 Ga. Therefore, an interaction of all Earth’s layers began from that time. 相似文献
12.
The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal-liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal-silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron-wüstite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor. 相似文献
13.
《Geochimica et cosmochimica acta》1999,63(13-14):2105-2122
We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature data are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composition of ALH84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity with EET79001-B for siderophile as well as lithophile elements; both are extraordinarily depleted in the “noblest” siderophiles (Os and Ir), to roughly 0.00001 × CI chondrites. As in terrestrial igneous rocks, among martian rocks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. For Mars, Re shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2–3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks generally show less clear-cut trends (relatively few MgO-rich samples are available). These trends are exploited to infer the compositions of the primitive Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth’s, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth’s mantle. This similarity constitutes a key constraint on the style of core-mantle differentiation in both Mars and Earth. Successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles (“high” compared to the lunar and HED mantles, and to models of simple partitioning at typical low-pressure magmatic temperatures), but only predict high Ni for the Earth’s mantle. Models that engender the noble siderophile excess in Earth’s mantle through a uniquely terrestrial process, such as a Moon-forming giant impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth’s size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final ∼ 1% of accretion these bodies acquired sufficient H2O to oxidize most of the later-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core. 相似文献
14.
The Moon has an anorthositic primordial continental crust. Recently anorthosite has also been discovered on the Martian surface. Although the occurrence of anorthosite is observed to be very limited in Earth's extant geological record,both lunar and Martian surface geology suggest that anorthosite may have comprised a primordial continent on the early Earth during the first 600 million years after its formation. We hypothesized that differences in the presence of an anorthositic continent on an Earthlike planet are due to planetary size. Earth likely lost its primordial anorthositic continent by tectonic erosion through subduction associated with a kind of proto-plate tectonics(PPT). In contrast, Mars and the Moon, as much smaller planetary bodies, did not lose much of their anorthositic continental crust because mantle convection had weakened and/or largely stopped, and with time, they had appropriately cooled down. Applying this same reasoning to a super-Earth exoplanet suggests that, while a primordial anorthositic continent may briefly form on its surface, such a continent will be likely transported into the deep mantle due to intense mantle convection immediately following its formation. The presence of a primordial continent on an Earth-like planet seems to be essential to whether the planet will be habitable to Earth-like life. The key role of the primordial continent is to provide the necessary and sufficient nutrients for the emergence and evolution of life. With the appearance of a "trinity" consisting of(1) an atmosphere,(2) an ocean, and(3) the primordial continental landmass, material circulation can be maintained to enable a "Habitable Trinity" environment that will permit the emergence of Earth-like life. Thus, with little likelihood of a persistent primordial continent, a super-Earth affords very little chance for Earth-like life to emerge. 相似文献
15.
Alan D. Brandon Thomas J. Lapen Brian L. Beard Clive Neal 《Geochimica et cosmochimica acta》2009,73(20):6421-17609
The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision 142Nd/144Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled 142Nd-143Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3 ppm (2σ) for 142Nd/144Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their 142Nd/144Nd isotope ratios in order to assess if the correlation line for 142Nd-143Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined 142Nd-143Nd-176Hf obtained we conclude that the 142Nd-143Nd correlation line measured in this study is best interpreted as an isochron with an age of 229+24−20Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44 Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm-Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth-Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth-Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required. 相似文献
16.
The tectono-magmatic evolution of the Earth and Moon started after the solidification of their magmatic “oceans”, whose in-situ
crystallization produced the primordial crusts of the planets, with the composition of these crusts depending on the depths
of the “oceans”. A principally important feature of the irreversible evolution of the planetary bodies, regardless of their
sizes and proportions of their metallic cores and silicate shells, was a fundamental change in the course of their tectono-magmatic
processes during intermediate evolutionary stages. Early in the geological evolution of the Earth and Moon, their magmatic
melts were highly magnesian and were derived from mantle sources depleted during the solidification of the magmatic “oceans”;
this situation can be described in terms of plume tectonics. Later, geochemically enriched basalts with high concentrations
of Fe, Ti, and incompatible elements became widespread. These rocks were typical of Phanerozoic within-plate magmatism. The
style of tectonic activity has also changed: plate tectonics became widespread at the Earth, and large depressions (maria)
started to develop at the Moon. The latter were characterized by a significantly thinned crust and basaltic magmatism. These
events are thought to have been related to mantle superplumes of the second generation (thermochemical), which are produced
(Dobretsov et al., 2001) at the boundary between the liquid core and silicate mantle owing to the accumulation of fluid at
this interface. Because of their lower density, these superplumes ascended higher than their precursors did, and the spreading
of their head parts resulted in active interaction with the superjacent thinned lithosphere and a change in the tectonic regime,
with the replacement of the primordial crust by the secondary basaltic one. This change took place at 2.3–2.0 Ga on the Earth
and at 4.2–3.9 Ga on the Moon. Analogous scenarios (with small differences) were also likely typical of Mars and Venus, whose
vast basaltic plains developed during their second evolutionary stages. The change in the style of tectonic-magmatic activity
was associated with important environmental changes on the surfaces of the planets, which gave rise to their secondary atmospheres.
The occurrence of a fundamental change in the tectono-magmatic evolution of the planetary bodies with the transition from
depleted to geochemically enriched melts implies that these planets were originally heterogeneous and had metal cores and
silicate shells enriched in the material of carbonaceous chondrites. The involvement of principally different material (that
had never before participated in these processes) in tectono-magmatic processes was possible only if these bodies were heated
from their outer to inner levels via the passage of a heating wave (zone) with the associated cooling of the outermost shells.
The early evolutionary stages of the planets, when the waves passed through their silicate mantles, were characterized by
the of development of super-plumes of the first generation. The metallic cores were the last to melt, and this processes brought
about the development of thermochemical super-plumes. 相似文献
17.
Presented are the review and analysis of a series of works of Russian scientists devoted to the motion of continents and to global tectonics and geodynamics. The increasing number of publications on this subject is accounted for by controversy and imperfection of some theoretical aspects of plate tectonics. This is especially true of processes in deep mantle geospheres. The analysis performed shows that a transition to creating tectonogeodynamic models of a new generation has already started. Important components of the models are the development of the problem of a detached continent drift, primarily, mechanisms of this motion. The publications under consideration provide insight into several such mechanisms: the effect of the tidal forces of the Moon and Sun, which induce discrete wave motion in all mantle geospheres down to the core; the difference in density of oceanic and continental masses; the continental drift over the mantle as a result of cohesion; and the tectonic flow, which implies a structure-forming motion of both minor (at the microlevel) and major rock masses in the Earth’s crust and mantle. The globalization of tectonic and geodynamic models has already allowed the advance of such a timely problem as the history of the geologic evolution of deep geospheres. 相似文献
18.
According to their genesis, meteorites are classified into heliocentric (which originate from the asteroid belt) and planetocentric
(which are fragments of the satellites of giant planets, including the Proto-Earth). Heliocentric meteorites (chondrites and
primitive meteorites genetically related to them) used in this study as a characteristic of initial phases of the origin of
the terrestrial planets. Synthesis of information on planetocentric meteorites (achondrites and iron meteorites) provides
the basis for a model for the genesis of the satellites of giant planets and the Moon. The origin and primary layering of
the Earth was initially analogously to that of planets of the HH chondritic type, as follows from similarities between the
Earth’s primary crust and mantle and the chondrules of Fe-richest chondrites. The development of the Earth’s mantle and crust
precluded its explosive breakup during the transition from its protoplanetary to planetary evolutionary stage, whereas chondritic
planets underwent explosive breakup into asteroids. Lunar silicate rocks are poorer in Fe than achondrites, and this is explained
in the model for the genesis of the Moon by the separation of a small metallic core, which sometime (at 3–4 Ga) induced the
planet’s magnetic field. Iron from this core was involved into the generation of lunar depressions (lunar maria) filled with
Fe- and Ti-rich rocks. In contrast to the parent planets of achondrites, the Moon has a olivine mantle, and this fact predetermined
the isotopically heavier oxygen isotopic composition of lunar rocks. This effect also predetermined the specifics of the Earth’s
rocks, whose oxygen became systematically isotopically heavier from the Precambrian to Paleozoic and Mesozoic in the course
of olivinization of the peridotite mantle, a processes that formed the so-called roots of continents. 相似文献
19.
Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale (x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations. 相似文献
20.
B. P. Kondratyev 《Astronomy Reports》2018,62(10):705-712
It is known that the center of mass (CM) of the Moon does not coincide with its geometrical center of figure (CF), and that the CF–CMline deviates to the Southeast of the direction toward the center of the Earth. An investigation of this phenomenon, which has remained incompletely understood, has been carried out in two stages. One mechanism can explain part of the eastward shift of the lunar CM as being due to tidal evolution of the lunar orbit. A second mechanism is considered here, which relates this shift of the lunar CM with evolution of the shape of the Moon. A differential equation describing the shift of the lunar CMto the East in the course of the physically inevitable rounding of its shape as it moves away from the Earth is derived and solved. This mechanism not only explains the eastward shift of the lunar CM, but also predicts that the oblateness of the Moon could have been appreciable at earlier epochs, reaching values ε ≈ 0.31. The theory of figures of equilibrium in a tidal gravitational field is used to determine how close to the Earth the Moon could have formed. 相似文献